Journal of Shipping and Ocean Engineering 6 (2016) 241-254
doi 10.17265/2159-5879/2016.04.006
Inelastic Nonlinear Pushover Analysis of Fixed
Jacket-Type Offshore Platform with Different Bracing
Systems Considering Soil-Structure Interaction
S. Ishwarya1, M. Arockiasamy
2 and R. Senthil
3
1. Division of Structural Engineering, Department of Civil Engineering, Anna University, Chennai, India
2. Department of Civil, Environmental and Geomatics Engineering, Florida Atlantic University, U.S.A.
3. Division of Structural Engineering, Department of Civil Engineering, Anna University, Chennai, India
Abstract: In this study, inelastic nonlinear pushover analysis is performed on a 3-D model of a jacket-type offshore platform for the
North Sea conditions. The structure is modelled, analyzed and designed using finite element software SACS (structural analysis
computer system). The behavior of jackets with different bracing systems under pushover analysis is examined. Further, by varying
the leg batter values of the platform, weight optimization is carried-out. Soil-structure interaction effect is considered in the analyses
and the results are compared with the hypothetical fixed-support end condition. Static and dynamic pushover analyses are performed
by using wave and seismic loads respectively. From the analyses, it is found that the optimum leg batter varies between 15 to 16 and
2% of weight saving is achieved. Moreover, it has been observed that the type of bracing does not play a major role in the seismic
design of jacket platform considering the soil-structure interaction.
Key words: Fixed offshore jacket platform, pushover analysis, seismic analysis, leg batter, optimization, SACS.
1. Introduction
Advances in reservoir assessment and recovery
techniques, subsea technology, seismic and directional
drilling techniques extend field life and impose higher
demands on existing offshore platforms to support
additional vertical and lateral loads. Jacket platforms
are fixed-base offshore structures that are used to
produce oil and gas in relatively shallow water,
generally less than 500 ft. These structures contribute
to a significant percentage of the world’s offshore
platforms. They are subjected to various
environmental loads during their life time. A
jacket-type platform consists of three parts: Jacket,
Pile and Topside. The primary function of a jacket
structure is to transfer the lateral loads to the
foundation and also support the weight of the topside
Corresponding author: M. Arockiasamy, Ph.D., P.E., P.
Eng., fellow ASCE, professor and director, research fields:
offshore technology, marine renewable energy.
structure. In seismically active areas, pile supported
structures may be subjected to strong ground motions
where the behavior of pile foundations under lateral
seismic loads becomes an important factor in
assessing the performance of such structures. Pile
foundations are an essential structural component of
jacket type offshore platform and the seismic
soil-pile-superstructure interaction is an important
concept in the seismic behavior. Strong ground
motions have been a major cause of past damage in
pile foundations.
Nonlinear static and dynamic analyses of fixed
offshore platforms have been carried out by many
researchers in last decades. Venkataramana et al. [1]
proposed a method for dynamic response analysis of
offshore structures subjected to simultaneous wave
and seismic loading and concluded that the sea waves
act as a damping medium and reduce the amplitude of
seismic response of offshore structures. Stear et al. [2]
validated the utility of static pushover analysis on two
D DAVID PUBLISHING
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
242
Gulf of Mexico platforms to assess its lateral load
capacity. It was concluded that the presence of
additional horizontal framing has little effect on the
first member failure in the jacket. Mwafy et al. [3]
assessed the applicability of static pushover analysis
by comparison with dynamic pushover analysis, by
developing complete pushover curves from
incremental dynamic analysis up to collapse for RC
(reinforced concrete) buildings. Mostafa et al. [4]
investigated the response of fixed offshore platforms
supported by clusters of piles and concluded that the
top soil layers have an important role in the response
of the structure. Nejad et al. [5] investigated the effect
of leg batter on overall cost of the Persian Gulf
platform and found the optimum batter as 11 which
lead to weight reduction of steel by 4%. Bargi et al.
(2011) performed nonlinear dynamic analysis of a
Persian Gulf platform and showed that the maximum
displacement response of platform under the
combination of earthquake and wave loads were more
than the maximum displacement response of
earthquake load alone. Bonessio et al. [6] formulated
an analytical model of BRBs (buckling restrained
braces) for structural optimization. Potty et al. [7]
assessed the ultimate strength of steel jacket offshore
platform under wave loads and found that X-bracing
contributes highest rigidity to the whole platform.
Nasseri et al. (2014) applied genetic algorithm for
optimizing the design of fixed offshore structure
subjected to environmental loads and determined the
contribution of member types in optimization. El-Din
et al. [8] evaluated the seismic performance of a
fixed-base jacket structure by nonlinear static analysis
and incremental dynamic analyses, and compared the
effects of various retrofit schemes. Nguyen et al. [9]
investigated the nonlinear behavior of a fixed-jacket
offshore platform with different bracing systems and
found that X-bracing system has a high seismic
performance.
Based on a simplified plastic collapse model, Lee
[10], has reported the piles in the system collapse
when one hinge forms at the pile head and a second
hinge forms at some depth below the pile head. The
collapse of the entire system occurs when two hinges
form in each of the piles in the system, as shown
schematically in Fig. 1.
A significant finding from the performance of
jacket platforms in major hurricanes, including Refs.
[11-15], is that pile foundations performed better than
expected [16-18]. While there are few cases with
“suspected” foundation damages or failures (Fig. 2),
there are no direct observations of such damages and
Fig. 1 Schematic diagram of pile system collapse due to
the formation of plastic hinges [10].
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
243
Fig. 2 Platform with “suspected” foundation failure [22].
failures during these hurricanes. Assessment of jacket
platforms subjected to environmental loads greater
than their original design loading frequently indicates
that the capacity of the structural system is governed
by the foundation. In addition, there were several
hundred platforms damaged in these hurricanes, and
yet only a few cases of pile foundation failures have
been reported. Therefore, there is a need to better
understand and quantify the potential conservatism in
foundation design for the purpose of assessing the
platforms. Reliable evaluation of the dynamic
response of jacket type offshore platform against
lateral loads plays an important role in the design.
From the literature, it is also evident that the
difference between the response of the jacket structure
with fixed support end condition and with PSI
(pile-soil-structure interaction), under static and
dynamic lateral loads, needs to be addressed.
In this study, at first, a typical four legged
jacket-type offshore platform is investigated for its
structural response by performing static and dynamic
wave response analyses. Then, time history
earthquake analysis is carried-out. Thirdly, inelastic
nonlinear static pushover analyses have been
performed using wave and seismic loads separately.
Further, weight optimization has been carried out by
analyzing different values of leg batter, varying from
3° to 14
° slopes. The analyses were performed using
finite element software, SACS, which are widely used
in practice for modelling soil-pile interaction.
2. Methodology
2.1 Three-Dimensional Finite Element Method Model
of Platform Structure
2.1.1 Description of 3-D FEM Model
The objective of the quantitative analysis is to
estimate the capacity of the foundation system.
SACS™ (structural analysis computer software) was
used to conduct 3-dimensional Finite Element Method
analyses of the case study platforms. SACS™ is a
suite of modular software developed by Engineering
Dynamics, Inc. for use in both offshore structures and
general civil engineering applications [19-21]. Use of
this software was made possible to the senior author
through the generous permission and support of NIOT
(National Institute of Ocean Technology), Chennai.
The inputs to this model are the structural properties
of all members and connections including the piles,
the behavior of the soil surrounding the piles (i.e., T-z
and p-y curves as a function of depth along each pile
and a Q-z curve at the tip), and the environmental
loading including the magnitude and direction of
waves and current. The primary output from this
model includes the total load on the structure,
typically expressed as a base shear, the displacement
of the deck, and the loads, moments and deformations
in individual members.
2.1.2 Theoretical Basis
The load-displacement relationship of a jacket
structure is determined using large deflection,
elasto-plastic, nonlinear, finite-element analysis. A
full plastic collapse (pushover) analysis is performed
to determine the load at which the structure collapses.
The solution process involves three levels of iteration.
For any global load increment, a beam-column
solution is performed for each plastic member using
the cross section sub-element details. The global
stiffness iteration is then performed including the
effects of connection flexibility, plasticity and failure
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
244
and the foundation stiffness iteration including the
nonlinear pile/soil effects. During any global solution
iteration, the deflected shape of the structure is
determined and compared to the displacements of the
previous solution iteration. If convergence is not
achieved, the new global displacements of the joints
along with the beam internal and external loads are
used to recalculate the elemental stiffness matrices.
The structural stiffness iteration is then repeated
including the effect of the foundation until the
displacements meet the convergence tolerance.
Non-linear pushover analysis. Static pushover
analysis is the application of a single load, applied to
any specific location which is incremented in steps
until collapse. COLLAPSE module is used to perform
pushover analysis of the jacket type offshore platform.
The COLLAPSE module employs elastic element, the
plasticity model and the failure criteria and uses basic
energy variation principles [23].
(1) The elastic element follows an updated
Lagrange (incremental iterative) procedure and uses a
nonlinear green strain formulation with the von
Karman approximation. The COLLAPSE beam
element is valid for large displacements and the
stiffness formulation derived from potential energy
consideration. Closed-form solutions of the nonlinear
elastic stiffness matrix are derived using the shape
function which represents the solution of the 4th order
differential equation for a beam-column.
(2) The plasticity model is represented by
concentrated yield hinges to reflect the nonlinear
material behavior. Hinges may be introduced at
element ends and/or element midspan. The plasticity
model is formulated in stress resultant (“force”) space
based on the bounding surface concept. Two
interaction surfaces are used, one yield surface
representing first fiber yield and another bounding
surface representing the full plastic capacity of the
cross section. The model allows for an explicit
formulation of the beam-element stiffness matrix,
including geometrical nonlinearities and nonlinear
plastic behavior with material hardening and gradual
plastification of the cross section.
(3) Failure is predicted in SACS COLLAPSE
module in accordance with failure criteria and design
formulation. The formation of a yield hinge is not the
limit of the load-carrying capacity and the structural
failure is indicated once a sufficient number of plastic
hinges have formed to make a kinematic mechanism.
The peak force in the member P-δ behavior defines
column buckling. The non-linear formulations
automatically calculate the total (1st + 2
nd order)
bending moments and also the effects of cross-section
geometry, boundary conditions and loading.
Pile-soil foundation models as per API RP2A
LRFD. Soil resistance is assessed at each node of the
pile segment and represented as a nonlinear spring.
The axial load transfer (T-z) normalized curves for
clay and sandy soils as recommended by API RP2A
LRFD are shown in Figs. 3 and 4. Fig. 5 shows the
unit end bearing capacity and normalized Q-z curve
for API clay and API sand. The ultimate lateral soil
resistance and P-y curves may be calculated using
sections G.8.2 to G.8.7 of API RP2A LRFD. The soil
resistance equations are not applicable, if the strength
Fig. 3 Normalized T-z curve for API clay [24].
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
245
Fig. 4 Normalized T-z curve for API sand [24].
Notation used for axis labels is defined as
t = mobilized skin friction;
tmax = maximum unit skin friction capacity (f);
z = local pile deflection;
D = diameter of pile.
Fig. 5 Normalized Q-z curve for API clay and API sand
[24].
Fig. 6 Normalized P-y curve for API clay ( for z < XR )
[24].
variation along the depth of the soil is inconsistent.
The ultimate unit lateral resistance, pu, of soft clay
under static loading conditions can vary between 8c to
12c (c: undrained shear strength of undisturbed clay
soil sample) except at the shallow depths. The P-y
curves of API clay under cyclic and static loading
conditions are defined as a piecewise linear function
and shown in Figs. 6 and 7.
Fig. 8 shows the 3D model of the jacket. The
platform considered in the study is a typical four
legged production platform made from tubular steel.
Water depth at the North Sea location considered is 75
m. Total height of the platform (jacket and deck) is 90
metres and has a square cross-section in plan. The
platform is designed based on the API RP 2A-WSD
and DNV standards. The major deck framing is 20 m
by 20 m in plan, dimension at the foot is 32 m by 32
m and the jacket legs are battered at 1 to 12.5
(horizontal to vertical) in both broad side and end
on framing. The jacket is subdivided into 3 bays of 25
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
246
Fig. 7 Normalized P-y curve for API clay (for z ≥ XR) [24].
Notation used for axis labels is defined as
P = actual lateral resistance in stress units;
Pu = ultimate lateral bearing capacity in stress units;
y = actual lateral deflection;
yc is defined as: yc = 2.5 εc D.
where εc = strain occurring at one-half the maximum stress on
laboratory undrained compression tests of undisturbed soil
samples and XR = depth from the ground surface to the bottom
of reduced resistance zone.
Fig. 8 SACS 3D model of jacket.
meters each with X-bracing. It is assumed that the
jacket structure is analyzed with the load from topside
acting as equivalent concentrated vertical load on the
piles and the wind effect is neglected as the jacket is
under water and wind effects are not considered in the
seismic analysis.
The different types of bracing configurations
considered in the study are shown in Fig. 9.
2.2 Material and Geometric Properties
Tables 1 and 2 show the material and geometric
properties of the jacket respectively. The length of the
pile is 100 meters below mud-line.
2.3 Loading Data
(1) Dead load
The total deck load of 4,800 tonnes is applied as
concentrated vertical load on the 4 piles.
(2) Wave and current load
Wave load has been generated using Stoke’s fifth
order equation. The maximum directional wave
heights for the 100-year return period are given in
Table 3.
Surface current velocity is considered as 1 m/s for
operating condition and 2 m/s for extreme condition.
(3) Soil data
The following (Table 4a) soil layering data are used
for the analysis. Soil properties vary with depth and
are characterized by the shear wave velocity Vs and
unit weight γ. Han et al. [25] have reported the soil
properties as shown in Table 4b.
(4) Load combinations
Fig. 9 Different types of bracing configuration.
Table 1 Material properties and mass of deck [9].
Parameter Value
Young’s modulus 2 108 kN/m2
Poisson’s ratio 0.3
Steel density 78.5 kN/m3
Yield stress 3.2 105 kN/m2
Ultimate stress 4 105 kN/m2
Mass of the deck 4,800 tonnes
Table 2 Geometric properties [9].
Member Diameter (mm) Thickness (mm)
Horizontal bracings and 1,500 20
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
247
diagonals
Diagonals (vertical plane) 1,600 30
Legs 2,000 50
Piles 1,900 40
Table 3 Wave load data [4].
Return period
(year)
Wave
height (H)
in meter
Height above
MSL (a) in
meter
Wave period
mean value (T)
in seconds
1 (operating
condition) 22.5 12.8 13.8
10 25.3 14.2 14.6
100 (extreme
condition) 28.5 16.1 15.3
10,000 36 20.4 17.1
Table 4a Soil layering data [4].
Soil unit Depth (m) Soil description
A 0-7.5 Very soft to soft silty, sandy clay
B 7.5-32 Sandy, clayey silt
C 32-47 Very stiff to hard silty clay
D 47-52 Very dense fine sand
E 52-125.5 Very stiff to hard clay
F >125.5 Very hard clay
Table 4b Soil properties.
Soil γ (kN/m3) Vs ( m/s)
Soft clay 18 130
Fine sand 18 140
Stiff clay 20 300
Silty sand 19 240
Silty clay 18 300
Shale 18 200
Sand 20 300
The analysis is done for both operating and extreme
conditions in eight directions as specified by the API
code (Fig. 10).
Fig. 10 Wave load directions (API guidelines).
3. Results and Discussions
3.1 In-place Analysis
In-place analysis is done to check the global
integrity of the structure against premature failure.
The following steps were performed in the in-place
analysis.
(1) Static analysis of the structure under wave load
acting in eight directions, as per API guidelines;
(2) Free vibration analysis to obtain natural time
period for corresponding modes;
(3) Dynamic wave response analysis in 8 directions,
using the significant wave height and time period.
3.1.1 Static Analysis
The combined utilization ratio check for the
members showed that the stresses are within allowable
limits and also the maximum utilization ratio (Eq. 1)
of critical members was lesser than one for both fixed
support condition and for PSI condition. The
maximum horizontal joint displacements were within
allowable limits (33% of length of the member) as per
API. The utilization ratio is defined as
(1)
3.1.2 Free Vibration Analysis
Free vibration analysis was performed to obtain
the natural time period of the structure.
The natural time periods of the structure in the 1st
and 2nd
modes were very close with a value of 2.7 s
under fixed support condition.
The natural time periods of the structure with PSI
condition for the 1st and 2
nd modes were similar to that
for the fixed condition with a value of 2.9 s.
3.1.3 Wave Response Analysis
Wave response analysis was carried out with P-M
wave spectrum (Eq. 2). Maximum hydrodynamic
force obtained was 7,880 kN in X-direction. Table 5
shows the results of wave response analysis.
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
248
(2)
where, α = 8.1 × 10−3 and is called the Phillips’
constant and B = 0.74(g)/2πU)4.
Wave response analysis results are given in Table 5.
3.2 Earthquake Analysis
Seismic analysis was carried out using El-Centro
Time History (Fig. 11) which has duration of 31.18
sec and maximum acceleration of 312.76 cm/s2.
The results of the earthquake analysis under both
fixed support condition and with PSI are given in
Table 6. The joint location considered for joint
displacement is shown in Fig. 12.
Table 5 Results of wave response analysis.
Response Fixed support PSI condition
Max. overturning
moment (kN m) 7.3 105 1.02 106
Max. base shear
(kN) 12,083 16,608
Fig. 11 El-Centro time history.
Table 6 Results of seismic analysis.
Response Fixed support PSI
Overturning
moment 4.2 106 kN m at
22.6 s
2.3 106 kN m at
23.4 s
Base shear 114,097 kN at 6.3 s 60,00 0 kN at 28 s
Joint displacement
401 L 42.9 cm at 26 s 28.3 cm at 23.6 s
Fig. 12 Wireframe 3D model in SACS.
Seismic analysis is performed by varying the
bracing configurations and the results are shown in
Table 7.
It is evident that the maximum responses are greatly
reduced by considering PSI under seismic analysis.
3.3 Pushover Analysis
The SACS program module COLLAPSE is used to
perform pushover analysis of the jacket-type offshore
platform. Pushover analysis is a static nonlinear
analysis method where a structure is subjected to
gravity loading and a monotonic displacement
controlled lateral load pattern which continuously
increases through elastic and inelastic behavior until
an ultimate condition is reached. In this study, two
separate pushover analyses were carried out, one with
wave load and another with seismic load.
3.3.1 Static Pushover Analysis Using Wave Load
Wave load in a particular direction is incremented
up to collapse of the structure. The base shear value at
formation of first hinge is not the limit of the load
carrying capacity. Pushover analysis is performed on
the four chosen bracing configurations under both
fixed and soil conditions. The results obtained in the
static pushover analysis using wave load are shown in
Figs. 13-19.
Important observations from the pushover analysis
are as follows:
(1) Base shear value at the formation of first hinge
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
249
is not the limit of the load-carrying capacity.
(2) The type of bracing does not play a major role
in the design of jacket considering soil-structure
interactions.
(3) It is economical to use a batter that gives
minimum weight of the structure and a realistic
analysis that takes into account the soil conditions.
3.3.2 Pushover Analysis Using Seismic Load
Pushover analysis of the Jacket type offshore
platform due to earthquake loads is performed to
ascertain the response and reserve strength against
future extreme intensity events. Pushover analysis
performed on X-bracing with leg batter value of
12.5 for fixed
Table 7 Seismic analysis on various bracing configurations.
X-bracing X+ Chevron
bracing
Chevron
bracing V-bracing X-bracing
X+ Chevron
bracing
Chevron
bracing V-bracing
[1] Fired support condition [2] PSI condition
Natural time
period (s) 2.61 2.86 2.84 2.62 2.92 3.15 3.11 2.91
Max. base shear
(kN)
53,661 at
11.2 s 48,212 at 6 s 47,818 at 6 s
44,741 kN at
7 s
63,857 kN at
10.3 s
53,252 at
13.6 s
47,405 at
13.5 s
56,785 kN at
11.9 s
Max. overturning
moment (kN m) 3106 kN m
at 5.7 s
3.25106 kN
m at 11.6 s
3.22106 kN
m at 11.6 s
2.81106 kN
m at 12.5 s
3.2106 kN m
at 11.6 s
2.33106 kN
m at 12.3 s
2.32106 kN
m at 13.8 s
3.24106 kN
m at 11.6 s
Max. joint
displacement
(joint 401 L)
(cm)
32 cm at 7 s 44 cm at
11.6 s
43.2 cm at
11.6 s
32.5 cm at
12.5 s
42 cm at 13.2
s
35.5 cm at
10.7 s
36.08 cm at
10.6 s
42 cm at 13.2
s
Weight (kN) 28,353.13 26,236.78 24,079.47 23,824.14 28,353.13 26,236.78 24,079.47 23,824.14
Fig. 13 Deformation stages due to pushover analysis on
X-bracing configuration-fixed end condition.
Fig. 14 Pushover curve on X-bracing configuration-fixed
end condition.
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
250
Fig. 15 Deformation stages due to pushover analysis on
X-bracing configuration-PSI.
Fig. 16 Pushover curve on X-bracing configuration-PSI.
Fig. 17 Ultimate strength of jacket with different bracing
configurations-fixed end condition.
Fig. 18 Ultimate strength of jacket with different bracing
configurations-PSI.
Fig. 19 Graphical representation of results for pushover
analysis with wave load.
end-support condition and with soil-structure
interaction. Equivalent seismic load was calculated
using the lumped mass values, peak acceleration value
and zone factor. Figs. 20-23 show the results obtained
by performing pushover analysis with seismic load.
Fig. 20 Deformation stages due to pushover analysis with
seismic load results-fixed end condition.
Fig. 21 Pushover curve on X-type bracing configuration
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
251
for seismic load-fixed end condition.
Fig. 22 Deformation stages due to pushover analysis with
seismic load results–PSI condition.
Fig. 23 Pushover curve on X-type bracing configuration
for seismic load-PSI condition.
Fig. 24 Jacket leg batter.
The maximum base shear at first hinge formation is
30% more in fixed end support structure in
comparison with structure with soil conditions.
3.4 Optimization of Leg Batter
In this study, the chosen jacket type platform is
investigated for optimum weight. The jacket leg batter
(Fig. 24) is an important parameter which influences
the jacket total weight. Therefore, eleven jackets with
a range of 3° to 14° (4 to 18 vertical to horizontal)
slope in leg batter have been modelled and analyzed
by a finite element program, SACS.
For optimization with variation of leg batter,
members are redesigned until stress ratio for critical
members in 11 jackets became equal to each other.
The optimum leg batter obtained is 16. Fig. 25 shows
the graphical representation of variation of weight
with leg batter and Fig. 26 shows the variation of
natural frequency of structure with leg batter.
The results of nonlinear static and dynamic analysis
of control structure and the optimized structure are
presented in Table 8.
Fig. 25 Optimisation of leg batter.
Fig. 26 Leg batter vs. Natural time period.
Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction
252
Table 8 Control structure vs. optimized structure results.
Analysis type Control structure (fix) Control structure (PSI) Optimized structure (fix) Optimized structure (PSI)
Weight 28,793 kN 28,178 kN
Free vibration
(time period) 2.61 s 2.95 s 2.75 s 3.13 s
Wave response
(base shear) 13,252 kN 9,795 kN 10,777 kN 10,552 kN
Response spectrum
(base shear) 46,000 kN 52,000 kN 54,595 kN 49,200 kN
Time history
(base shear) 53,661 kN 63,857 kN 50,000 kN 51,442 kN
Pushover wave
(base shear) 48,000 kN 32,000 kN 52,000 kN 39,000 kN
Pushover seismic
(base shear) 43,000 kN 33,000 kN 63,000 kN 41,000 kN
4. Conclusions
The following conclusions are drawn based on the
analytical investigations carried out using SACS to
study the nonlinear static and dynamic response of
offshore jacket structure.
(1) The natural frequency of the structure is around
2.9 seconds which is well within the range for fixed
offshore structures.
(2) The maximum base shear obtained from
dynamic wave response analysis for fixed support
condition is about 35% more than that of analysis with
soil-structure interaction.
(3) Base shear and overturning moment values due
to earthquake forces for jacket with soil condition are
about 25% more in time history analysis than response
spectrum method. The response spectrum method is
an approximate method used to estimate maximum
peak values of displacements and forces and it has
significant limitations. It is restricted to linear elastic
analysis in which the damping properties can only be
estimated with a low degree of confidence.
(4) The pushover analysis on the jacket structure
shows that it possesses strength substantially in excess
of original design loads.
(5) Base shear value at formation of first hinge is
not the limit of the load-carrying capacity and
structural failure is indicated once a sufficient number
of plastic hinges have formed to make a kinematic
mechanism.
(6) The type of bracing does not play a major role
in the seismic design of jacket considering
soil-structure interaction since the first member failure
occurs in almost similar base shear values for jacket
structure with various bracing conditions.
(7) The ultimate strength of jacket structure under
seismic load and wave load is nearly the same with
marginal variation of 5%.
(8) The weight of the chosen jacket has been
reduced by 2% with optimized leg batter of 16.
(9) The optimized structure has more flexibility and
has 25% more ultimate strength capacity in
comparison with the actual control structure.
Acknowledgements
The authors are grateful for the facilities and
support provided by Florida Atlantic University,
U.S.A. and National Institute of Ocean Technology,
Chennai, India for granting permission to have access
and to use the SACS software.
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